There has been generally known a cylinder head gasket held between a cylinder head and a cylinder block having a cylinder bore, and including a plurality of seal plates each having a combustion chamber hole formed at the position of the cylinder bore.
A cylinder head gasket adapted to seal a gap between a cylinder head and a cylinder block by use of a self sealing effect is known as this type of cylinder head gasket (Patent Document 1).
To be more specific, in the cylinder head gasket according to Patent Document 1, a projecting portion that projects toward the cylinder head around the combustion chamber hole is formed on a seal plate on the cylinder head side, and a radial center portion of the projecting portion is formed projecting toward the cylinder block. Also, a projecting portion that projects toward the cylinder block around the combustion chamber hole is formed on a seal plate on the cylinder block side, and a radial center portion of the projecting portion is formed projecting toward the cylinder head. The seal plates overlap each other such that the radial center portions of the both projecting portions are brought into close contact with each other in a circumferentially continuous manner.
Since the radial center portions of the projecting portions are in close contact with each other in a circumferentially continuous manner in the cylinder head gasket having the aforementioned configuration, a combustion gas to be blown out through a gap between the radial center portions presses one of the projecting portions toward the cylinder head, and also presses the other projecting portion toward the cylinder block by its pressure.
When one of the projecting portions is pressed against the cylinder head by the pressure of the combustion gas, the sealability of the pressed portion is improved. When the other projecting portion is pressed against the cylinder block by the pressure of the combustion gas, the sealability of the pressed portion is also improved.
Moreover, when one of the projecting portions is pressed against the cylinder head and the other projecting portion is pressed against the cylinder block, the radial center portion of one of the projecting portions and the radial center portion of the other projecting portion are brought into closer contact with each other by the reactions. Thus, the sealability of the portion is also improved.
That is, with the cylinder head gasket according to Patent Document 1, a good sealing effect can be obtained from the self sealing effect by use of the pressure of the combustion gas.    Patent Document 1: Japanese Patent Laid-Open No. 2000-88105